You can transfer the condo directly from the Thai seller into your name and pay the money to the unregistered farang owner. The land office needs to see the letter from your bank saying you bought the money in to purchase a condo. You do not have give details of the condo in the bank letter or who you are buying it from it just needs to state you bought the money in to buy a condo. You obviously need to cooperate with the registered owner and the farang unregistered owner with both attending the settlement at the land office but you will just waste money by transferring it into the foreigners name then immediately transferring into your name as taxes and stamp duty will need to be paid twice. You will probably need to organize cashiers checks for the unregistered seller, any amount owed to the registered seller, and the tax/stamp duty. The seller may want cash for the stamp duty and tax. The seller should give you a breakdown. if you are not confident get a property lawyer to represent you. It will cost you 20-30k ( maybe cheaper if you are not in Bangkok ) but it will get done.